My site is hacked

Hi
I am just a newbie and i made a website for my department with wordpress. https://cusatnasb.inRecently i used w3 Total cache and found that in certain pages i found porn ads linking to my site. I checked the cache folder in wp-contents in my server and found that the bad links were there in the index.html file. I edited the file and reactivated the plugin and the problem was solved.But it aganin appeared a few days later. so i have permanently disabled w3 total cache . My doubt is how is that link coming there. Possibilites of my file manager access is zero as i check the logs often. So how is the bad links coming in the html files. I have tried reinstalling w3 total cache but the problem persists.

Can Some one please help me. It is very urgent as the site is for a educational deparment.